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Journal of Alloys and Compounds 338 (2002) 87–92
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Synthesis, characterization and bonding of Ba 3 Li 4 Sn 8
Svilen Bobev, Slavi C. Sevov*
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA
Received 18 October 2001; accepted 26 November 2001
Abstract
The ternary compound Ba 3 Li 4 Sn 8 was obtained by slow cooling of a stoichiometric mixture of the elements that was heated at 800 8C
˚ V5833.2(2) A
˚ 3 , R 1 /wR 2 5 4.03 / 11.33%)
for 2 days. Its structure (monoclinic, C2 /m, Z52, a518.549(1), b56.685(1), c56.792(1) A,
1
102
contains one-dimensional chains of ` [Sn 8 ]
made of edge-sharing cyclodecane-like units in their ‘all-chair’ conformations. The cations
screen the chains from each other and provide the extra electrons needed for bonding in the chains. The structure and the electron count
can be rationalized in terms of the Zintl concept as (Ba 21 ) 3 (Li 1 ) 4 [(2b-Sn 22) 2 (3b-Sn 12) 6 ], i.e. the compound is an electronically
balanced, Zintl phase. Nevertheless, Ba 3 Li 4 Sn 8 exhibits temperature-independent paramagnetism that is, most likely, of van Vleck type as
a result of the very small band gap of about 0.3 eV.  2002 Elsevier Science B.V. All rights reserved.
Keywords: Intermetallic; Zintl phases; X-ray diffraction; Electronic band structure
1. Introduction
The discovery of Tt 42
in neat solids (Tt5Tetrel,
9
] ]
element of group 14) stimulated the search for Zintl phases
with other isolated clusters, either new or similar to those
known as crystallized from solutions [1–6]. Our thorough
and systematic studies of the binary A–Tt systems (A5
alkali metal) extended into the pseudo-binary systems with
mixed alkali metals, especially combinations of very
different sizes, and provided the first arachno-clusters of
group 14, Sn 62
in A 4 Li 2 Sn 8 for A5K or Rb [7]. Similar8
ly, germanium forms novel giant clusters, truncated tetrahedra of Ge 122
12 , when combined with a mixture of Li and
Rb in RbLi 7 Ge 8 [8]. It is important to emphasize that both
novel species form only in Li-containing, mixed alkalimetal phases, and not in simple binaries. Besides the role
of the sizes of the cations these phases are apparently
stabilized also by the better covalency of lithium which
caps the open faces of these clusters and stabilizes them as
closo-species, Li 2 Sn 42
and Li 4 Ge 82
8
12 [7,8]. Also, mixed
alkali metals provide compounds with isolated clusters of
42
lead, Pb 42
of different shapes [9].
4 -tetrahedra and Pb 9
Following the studies of the pseudo-binary systems with
mixed alkali metals we have taken one step further in
complexity and have studied the systems alkali-metal (A)–
alkaline-earth metal (AE)–tetrel (Tt). Found in these
systems were truncated tetrahedra of Sn 122
in
12
[AE]Na 10 Sn 12 for AE5Ca or Sr [10], and similar to the
82
lithium-stabilized closo-Li 4 Ge 12
, they are stabilized by
82
sodium as closo-Na 4 Sn 12 clusters. Using Ba instead of Ca
or Sr led to the formation of the largest naked cluster of a
main-group element, Sn 442
in Ba 16 Na 204 Sn 310 [11]. This
56
cluster is made of four fused pentagonal dodecahedra of tin
that are stuffed with four barium cations. Here we report
the synthesis and structural characterization of another
compound with mixed alkali and alkaline-earth cations,
Ba 3 Li 4 Sn 8 . It exhibits one-dimensional chains of
1
102
that are made of edge-sharing cyclodecane-like
` [Sn 8 ]
units in their ‘all-chair’ conformation, unprecedented structural motifs in the chemistry of the heavier carbon homologues.
2. Experimental section
2.1. Synthesis
*Corresponding author. Tel.: 11-219-631-5891; fax: 11-219-6316652.
E-mail address: [email protected] (S.C. Sevov).
All manipulations were performed in an Ar-filled glove
box. The surfaces of Li (rod, Aldrich, 99.9%, sealed under
0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved.
PII: S0925-8388( 02 )00219-0
88
S. Bobev, S.C. Sevov / Journal of Alloys and Compounds 338 (2002) 87 – 92
Ar) and Ba (rod, Alfa, 99.2%, sealed under Ar) were
cleaned off with a scalpel immediately before use, while
Sn (rod, Alfa, 99.999%) was used as received. All reactions were carried out in sealed niobium tubes, which
were in turn jacketed and sealed under vacuum in silica
ampoules. More details on the techniques and the procedures are described elsewhere [12].
Initially, Ba 3 Li 4 Sn 8 was synthesized as the major
product of a reaction designed to produce BaLi 10 Sn 12 , the
barium–lithium analog of [AE]Na 10 Sn 12 (AE5Ca, Sr)
[10]. The reaction was carried out at 800 8C (m.p. of Ba is
725 8C; no phase diagram for Ba–Sn; the highest m.p. in
Li–Sn is 780 8C) for 48 h and cooled down to room
temperature with a rate of 5 8C / h. Other products from that
reaction, as identified by powder X-ray diffraction (EnrafNonius Guinier camera with a vacuum chamber and Cu
Ka radiation), were traces of BaSn 3 and LiSn as well as
some unreacted Sn metal. After the stoichiometry of the
compound was obtained from the structure determination,
stoichiometric mixtures were treated at the same temperature regimes. A least-squares refinement of the measured
2u values of the X-ray powder diffraction pattern of the
compound relative to those of the internal standard
(silicon) verified the lattice constants.
2.2. Structure determination
Numerous crystals of Ba 3 Li 4 Sn 8 were selected in the
glove box and were sealed in 0.2 mm thin-walled glass
capillaries. They were checked for singularity on an EnrafNonius CAD4 single-crystal diffractometer (graphite˚ One of
monochromated Mo Ka radiation, l 50.71073 A).
these crystals (irregular, 0.1630.1430.08 mm) was
chosen and a unique quadrant of X-ray diffraction data was
collected at room temperature with v –2u scans and up to
2u of 608. All 2013 reflections were corrected for absorption with the aid of the average of c-scans of four
reflections at different u angles (R int 50.051; T min /T max 5
0.154 / 0.316). The observed extinction conditions and the
intensity statistics suggested the centrosymmetric space
group C2 /m. Consequently, the structure was successfully
solved and refined with the aid of the SHELXTL-V5.1
software package [13] in that space group. The direct
methods provided three peaks with distances to each other
appropriate for tin atoms, and two peaks with distances to
the other three that were appropriate for barium atoms, and
they were all so assigned. The following least-squares
cycles and a difference Fourier synthesis revealed two
peaks at special positions and with relatively low heights.
These were assigned as lithiums, and the subsequent
anisotropic refinement of all atoms converged at R 1 5
4.17%; wR 2 5 11.43% for 1309 independent reflections
and 43 parameters. Further details on the data collection
and refinement parameters are summarized in Table 1,
while positional and equivalent isotropic displacement
Table 1
Selected data collection and refinement parameters for Ba 3 Li 4 Sn 8
Chemical formula
Formula weight
Space group, Z
˚
Unit cell parameters (A)
Ba 3 Li 4 Sn 8
1389.30
C2 /m (No. 12), 2
a518.549(1)
b56.685(1)
c56.792(1)
b 598.34(1)8
˚3
V 5 833.2(2) A
Mo Ka, 0.71073
608
186.84
5.538
1309 / 0 / 43
R 1 5 4.03%, wR 2 5 11.33%
R 1 5 4.17%, wR 2 5 11.43%
˚
Radiation, l (A)
2u limit
m (cm 21 )
rcalcd (g / cm 3 )
Data / restraints / parameters
R indices (I . 2sI )a
R indices (all data)
a
R 1 5 ouuFo u 2 uFc uu / ouFo u; wR 2 5 [o[w(F o2 2 F c2 )2 ] / o[w(F o2 )2 ]] 1 / 2 . w 5
1 / [s 2 F o2 1 (0.0673P)2 1 29.333P], P 5 (F o2 1 2F c2 ) / 3.
parameters and important distances are listed in Tables 2
and 3, respectively.
2.3. Magnetic measurements
The magnetizations of different samples of Ba 3 Li 4 Sn 8
were measured on a Quantum Design MPMS SQUID
magnetometer at a field of 30 kG over the temperature
range 10–280 K. Measurements at lower temperatures
(down to 4 K) and lower magnetic fields of 50, 100 and
300 G were carried out as well. The samples were
contained in special holders designed for air-sensitive
compounds [12]. The raw data were corrected for the
holder contribution and for ion-core diamagnetism. The
corrected magnetic susceptibility is positive and temperature independent, and varies between 4.17310 24 and
4.98310 24 emu / mol (average of two measurements on
two different samples). No superconductivity was observed
at the low temperatures and magnetic fields.
2.4. Band calculations
¨
Extended Huckel
band calculations were carried out
within the tight binding approximation with only the tin
atoms included (Hii and z1 for Sn 5s: 216.16 eV and 2.12,
for Sn 5p: 28.32 eV and 1.82) [14]. The density of states
Table 2
Atomic coordinates and equivalent isotropic displacement parameters
˚ 2 ) for Ba 3 Li 4 Sn 8
(A
Atom
Site
x
y
z
Ueq
Sn1
Sn2
Sn3
Ba1
Ba2
Li1
Li2
4i
4i
8j
2a
4i
4h
4i
0.12920(4)
0.48483(4)
0.85055(3)
0
0.32245(4)
0
0.209(1)
0
0
0.73248(8)
0
0
0.787(4)
0
0.4541(1)
0.2020(1)
0.19789(8)
0
0.6913(1)
1/2
0.130(4)
0.0142(2)
0.0145(2)
0.0141(2)
0.0135(2)
0.0153(2)
0.029(5)
0.022(4)
S. Bobev, S.C. Sevov / Journal of Alloys and Compounds 338 (2002) 87 – 92
89
Table 3
˚ and angles (8) in Ba 3 Li 4 Sn 8
Important bond distances (A)
Sn1–
23
23
23
Sn2–
23
23
23
Sn3–
Sn3–Sn1–Sn3
Sn2–Sn2–Sn3
Sn3–Sn2–Sn3
Sn3
Li1
Li2
Ba1
Ba2
Ba2
Sn2
Sn3
Li1
Ba1
Ba2
Sn1
Sn2
Sn3
Li1
Li2
Li2
Ba1
Ba2
Ba2
Ba2
2.9440(9)
2.84(1)
2.82(2)
3.6211(9)
3.6342(6)
3.711(1)
2.880(2)
2.9325(8)
2.77(2)
3.6393(6)
3.543(1)
2.9440(9)
2.9325(8)
3.108(1)
3.221(3)
2.94(2)
3.03(2)
3.7107(6)
3.7422(9)
3.7977(9)
3.8421(8)
74.81(3)
105.90(4)
64.00(3)
Li1–
23
23
23
23
Li2–
Ba1–
Ba2–
23
23
23
23
43
23
43
23
23
23
23
23
Sn2–Sn3–Sn1
Sn2–Sn3–Sn3
Sn1–Sn3–Sn3
Sn1
Sn2
Sn3
Li1
Ba1
Sn1
Sn3
Sn3
Ba2
Sn1
Sn2
Sn3
Li1
Sn1
Sn1
Sn2
Sn3
Sn3
Sn3
Li2
2.84(1)
2.77(2)
3.221(3)
2.85(6)
3.68(1)
2.82(2)
2.94(2)
3.03(2)
3.630(9)
3.6211(9)
3.6393(6)
3.7107(6)
3.68(1)
3.6342(6)
3.711(1)
3.543(1)
3.7422(9)
3.7977(9)
3.8421(8)
3.630(9)
107.75(3)
58.00(1)
127.40(2)
and the crystal orbital overlap populations were calculated
on the basis of 240 k-points over the irreducible wedge of
the Brillouin zone.
3. Results and discussion
Fig. 1. General view of the C-centered monoclinic structure of Ba 3 Li 4 Sn 8
viewed: (a) along the b axis, and (b) along the c axis. The large and small
isolated spheres are Ba and Li atoms, respectively. The unit cell is
outlined.
3.1. Structure description
The structure of Ba 3 Li 4 Sn 8 consists of isolated chains of
tin that run along the b axis of the monoclinic cell (Fig. 1).
The repeating monomeric unit of the chains is cyclodecane
in its ‘all-chair’ conformation (Fig. 2). Each monomer
shares two directly opposite edges with its two neighbors
in the chain. Interestingly, there is another compound in
the ternary (AE)–Li–Tt system with almost the same
composition, Ca 31x Li 42x Si 8 [15], and its structure, although different from Ba 3 Li 4 Sn 8 , contains also chains of
the tetrel. The difference is that the chains of silicon are
made of interconnected xylene-like rings rather than fused
cyclodecanes. The cyclodecanes in Ba 3 Li 4 Sn 8 are made of
three unique tin sites (Fig. 2), two of which, Sn2 and Sn3,
are three-bonded, while the third, Sn1, is two-bonded. The
Sn–Sn distances within the cyclodecane monomer are
˚ (Table 3). They are
quite similar, 2.880(2)–2.9440(9) A
typical for single, localized Sn–Sn bonds such as those
found in Sn 42
tetrahedra and 1` [Sn] 22 zigzag chains [16].
4
The edges that are shared with neighboring cyclodecanes
are Sn2–Sn2 pairs. This brings the Sn3 atoms (the atoms
next to the shared edges) of different cyclodecanes in
˚ Thus, a triangle of one
relatively close proximity, 3.108 A.
Sn2 and two Sn3 atoms is formed when the Sn3–Sn3
Fig. 2. An ORTEP drawing (80% probability thermal ellipsoids) of a
three-monomer section of the 1` [Sn 8 ] 102 chain. The monomer is with the
shape of a cyclodecane in its ‘all-chair’ conformation (outlined). Each
cyclodecane shares two opposite edges with two neighbors.
90
S. Bobev, S.C. Sevov / Journal of Alloys and Compounds 338 (2002) 87 – 92
˚ is considered. This distance is
contact of 3.108(1) A
somewhat longer but compares well with the distances in
˚ [16], in BaSn 3 with
metallic b-Sn, 3.016 and 3.175 A
˚ [17], and in
isolated triangular Sn 322 units, 3.058 A
122
Ca 31 Sn 20 with Sn 62
dimers
and
Sn
linear
pentamers,
2
5
˚ [18], and should be considered in the
3.063–3.158 A
bonding picture. Longer distances in lithium-containing
compounds have been discussed extensively elsewhere
˚ relative
[19]. The elongation, usually in the order of 0.15 A
to Sn–Sn single-bond distance, is explained by the strong
cation field effects and strong polarizing power of lithium
[19].
The angles at the tin atoms deviate substantially from
the ideal tetrahedral angle and are in a very wide range, all
the way from 58.00(1) to 107.75(3)8. As might be
expected, the sharpest angles are within the isosceles
triangle of two Sn3 and one Sn2 atoms, 58.00(1) and
64.00(3)8. These are followed by the angle at the twobonded Sn1, 74.81(3)8, for which a relatively small angle
is normal due to the two lone pairs of electrons associated
with it. The remaining angles are closer to tetrahedral. It
should be pointed out that isolated triangular units of Sn 22
3
with two delocalized p electrons that are isoelectronic with
the aromatic cyclopropenium cation C 3 H 31 exist in BaSn 3
[18].
3.2. Electronic structure
The electron needs for bonding in the chains can be
discerned based on the octet rule. Thus, a three-bonded
pyramidal atom such as Sn2 and Sn3 carries a lone pair of
electrons and must provide an electron for each bond, or a
total of five electrons per atom. A two-bonded atom such
as Sn1 carries two lone pairs of electrons in addition to the
two electrons of the two bonds, or a total of six electrons.
This means that the formal oxidation state for Sn2 and Sn3
is 12 while for Sn1 it is 22. Thus, the need of additional
electrons for the bonding of the repeating unit of two Sn1,
two Sn2, and four Sn3 atoms becomes 23(22)123(12)
143(12)5102. Exactly that many electrons are available from the three Ba and four Li atoms in the formula of
the compound, Ba 3 Li 4 Sn 8 , and therefore the compound
should be electronically-balanced, or a Zintl phase.
This simple counting scheme assumes, of course, complete electron transfer from the alkali and alkaline-earth
metals to the post-transition element. However, it does not
provide insight into the electron distribution among the
different Sn–Sn bonds or the strength of the different
interactions. Furthermore, actual negative charges rarely
correspond to the assigned formal oxidation states. In order
to study the bonding of Ba 3 Li 4 Sn 8 in more detail and to
eventually relate its structure to the observed temperature¨
independent paramagnetism, extended Huckel
band calculations were carried out for the Sn sublattice. The total
density of states (DOS) is shown in Fig. 3 together with
Fig. 3. Shown are the total DOS (black line), the projected partial DOS
of Sn3 (gray line), and the Fermi level (broken line) for Ba 3 Li 4 Sn 8 . A
very small band gap of less than 0.3 eV separates the valence and
conduction bands.
the projected partial DOS of Sn3. The total of 42 electrons
for the repeating unit Sn 102
fill the bands below the Fermi
8
level at 25.12 eV. Thus, the valence band is completely
filled and is separated by a very narrow gap of less than
0.3 eV from the conduction band. This suggests that the
observed temperature-independent paramagnetism of
Ba 3 Li 4 Sn 8 is either of van Vleck type, i.e. paramagnetism
due to mixing of the ground state (filled valence band)
with thermally-empty, yet low-lying, excited paramagnetic
states (conduction band), or that simply the conduction and
valence bands overlap. The latter is quite possible because,
¨
as it is well known, extended-Huckel
calculations are only
an approximation and often overestimate the band gaps.
Furthermore, we should keep in mind that the calculations
did not include the cation–anion interactions, which will
further perturb the overall widths and positions of the
valence and conduction bands, and may very well cause
overlap between them. These interactions, especially with
lithium may have significant covalent character as already
pointed out.
S. Bobev, S.C. Sevov / Journal of Alloys and Compounds 338 (2002) 87 – 92
Closer look at the DOS (Fig. 3) indicates that, as
expected, the bands below 212.8 eV are the tin s states
while the valence band, between 25.12 and 211.2 eV, is
predominantly tin p states. In order to evaluate further the
bonding in this compound, we calculated crystal orbital
overlap population (COOP) curves for the different Sn–Sn
interactions (Fig. 4). They showed that the main bonding
states occur within the largest peak of the valence band in
the DOS, between 211 and 28 eV, while the states of the
same band above 28 eV are predominantly nonbonding,
i.e. lone-pair like, as should be. The states above the Fermi
level are clearly antibonding for all interactions around
Sn3 (Fig. 4b–d) while for the Sn2–Sn2 interactions they
remain nonbonding. The lowest-lying antibonding states
91
˚ for
are for the longest bond in the structure, 3.108(1) A
Sn3–Sn3 (Fig. 4d).
4. Conclusions
A new ternary Zintl phase, Ba 3 Li 4 Sn 8 , has been synthesized and structurally characterized. Its structure consists of chains made of edge-sharing cyclodecane-like units
of tin in their ‘all-chair’ conformations. The Li 1 and Ba 21
cations screen the chains from each other and provide the
extra electrons needed for chemical bonding and the
compound is electronically balanced. However, the compound shows temperature-independent paramagnetism,
Fig. 4. COOP curves for the four different Sn–Sn bonds in Ba 3 Li 4 Sn 8 : (a) Sn2–Sn2; (b) Sn1–Sn3; (c) Sn2–Sn3; (d) Sn3–Sn3.
92
S. Bobev, S.C. Sevov / Journal of Alloys and Compounds 338 (2002) 87 – 92
which is most likely due to either van Vleck paramagnetism due to a very small band gap of 0.3 eV, or because of
actual overlap of the conduction and valence bands.
[4]
[5]
[6]
[7]
[8]
[9]
Acknowledgements
We thank the Petroleum Research Fund, administered by
the ACS, for the financial support of this research.
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